1. Field of the Invention
The present invention relates to well servicing fluids and, more particularly, to aqueous well drilling fluids containing partially hydrolyzed polyacrylamides.
2. Description of the Background
In the drilling of oil and gas wells and other earth boreholes, a fluid, commonly referred to as mud, is pumped down through the drill pipe and returns up the annulus between the drill pipe and the borehole, the mud serving the dual purpose of cooling the bit and removing cuttings as the drilling proceeds. It is common to employ drilling fluids or muds which contain various substances which will form a thin barrier or coating on the well bore to prevent the hole from sloughing. Additionally, this thin cake or coating on the well bore prevents loss of the drilling mud or fluid out of the well bore into the adjoining formation. The problem of sloughing and concomitant fluid loss is particularly acute in the case of formations which contain shale. Shale formations, when contacted with aqueous drilling or other well servicing fluids, undergo heaving leading to sloughing of the well bore, resultant erosion and loss of drilling fluid into the formation.
It is well known that a drilling or well servicing fluid containing partially hydrolyzed polyacrylamide (PHPA) is quite effective in overcoming the problems encountered in shale formations. PHPA, when in contact with water, hydrates immediately, the hydrated polymer molecules absorbing onto the surfaces of both the cuttings and the well bore, thereby forming a stationary, thin coating. This coating prevents hydration of the cuttings permitting their removal at the surface with conventional shale shakers and other solids separation equipment. Additionally, the coating formed by the PHPA on the well bore surface prevents the active shale from sloughing and the well bore from erosion. However, it is known that as the drilling continues, the PHPA concentration in the drilling fluid is depleted due to the fact that there are new surfaces of cuttings and well bore for coating with the PHPA. Accordingly, it is generally necessary to monitor and periodically add additional quantities of PHPA to the drilling fluid.
In addition to the loss of PHPA in the drilling fluid by absorption on new cuttings and new exposed well bore, the PHPA can be destroyed by high solution pH and interaction with multiple valence cations, such as calcium, magnesium, etc. High solution pH values cause the PHPA polymers to hydrate to polyacrylate salts, while it is believed that multivalent cations, e.g. calcium, cause the PHPA molecules to cross-link or to precipitate out of the drilling fluid. It is known that when PHPA containing drilling fluids are contaminated with excess calcium ion, the fluid becomes viscous and the ability of the PHPA to decrease fluid loss decreases dramatically. While the loss of PHPA by absorption on new cuttings or well bore can be overcome simply by adding more PHPA to the drilling mud, the destruction of PHPA due to excess calcium or other multivalent cations can only effectively be prevented by preventing or at least interfering with the reaction between the calcium or other such ions and the PHPA.
In a typical oil or gas well, excess calcium ions can be encountered either from calcium enriched formations encountered during the drilling operations or from cementing operations, e.g. the setting of casing. For example, it is common to use cementing operations to isolate various formations vertically disposed in the well bore. In these cases, once a cementing operation has been completed, it is necessary to drill through the cement column which inherently exposes the drilling fluid to high calcium leaching from the cementing cuttings. Accordingly, it would be desirable to have an aqueous-based drilling or well servicing fluid containing partially hydrolyzed polyacrylamides which will be stable in the presence of calcium and other similar multivalent cations.